Photon Science Division, Paul Scherrer Institut, Switzerland
X-ray free-electron lasers (FEL) provide coherent femtosecond x-ray pulses with unprecedented brilliance, enabling x-ray scattering measurements that are extremely challenging at synchrotrons. First, I will report on the status of the SwissFEL project at the Paul Scherrer Institut. Last year SwissFEL has seen first light and pilot experiments are expected from the end of this year on. Then, I will illustrate the capabilities of x-ray FELs at the example of two studies carried out at the Linac Coherent Light Source (LCLS):
Synchronization of a pulsed magnet with the bright x-ray FEL pulses reveals that a three-dimensional charge-density wave emerges at high fields in the superconducting cuprate YBa2Cu3O6+δ. This approach enables x-ray scattering at magnetic field strengths up to 32 Tesla, comparable to those used in other high-field techniques.
The shortness of the x-ray FEL pulses results also in an outstanding time-resolution in, so-called, pump-probe experiments. I will describe how time-resolved x-ray scattering can quantify the lattice dynamics in iron-based compounds upon coherent excitation of an optical phonon. In combination with time- and angle-resolve photoemission spectroscopy this scheme enables purely experimental precision measurements of fundamental physical quantities, such as the electron-phonon deformation potential in FeSe.